The present invention relates to a cam structure equipped with a roller and constituting a valve mechanism of an engine.
A cam structure equipped with a roller is known as a type of a cam constituting a valve mechanism of an engine. For example, JP-A-2011-80372 and JP-A-2012-202355 disclose cam structures in each of which a roller is mounted on a base cam having a base circular section and a valve lift section. The roller is provided in a cut-out section formed at the tip end of the valve lift section and mounted so that part of its outer peripheral face protrudes outward from the outer peripheral face of the valve lift section.
This type of cam equipped with a roller is mounted on a camshaft, rotates together with the camshaft in synchronization with the crankshaft of an engine, and drives a tappet provided at the base end section of a valve. With the rotation of the camshaft, the valve lift section of the cam equipped with the roller first makes contact with the tappet to press the tappet. Then, the contact position with the tappet is shifted from the valve lift section to the roller, and the roller presses the tappet this time. The roller presses the tappet while rotating on the tappet.
With this configuration, it is assumed that the following excellent advantages are obtained; since the roller of the cam equipped with the roller presses the tappet while moving and rolling on the tappet, the friction between the cam and the tappet can be reduced and fuel economy can be improved in comparison with a cam with no roller, and since the roller itself rotates on the tappet, the torque for driving the cam at a low-rotation region can be reduced.
However, in a type of tappet driven by a cam, a spherical crowning is formed on the face (top face) of the tappet making contact with the cam. In addition, in a type of cam, a crowning is formed on the face (cam face) of the cam making contact with a tappet. For example, JP-A-2011-117415 discloses a valve mechanism in which a crowning is formed on each of the top face of a tappet and the face of a cam. It is assumed that friction increase and the occurrence of uneven abrasion due to misalignment can be suppressed by properly setting the amount and the radius of curvature of each crowning.
However, in the case that a crowning is formed on each of the top face of the tappet and the face of the cam as disclosed in JP-A-2011-117415 described above, an axial thrust load may be generated on the cam depending on the contact position between the tappet and the cam, and the friction therebetween may increase. In particular, in the case of a cam structure with a roller, if an axial thrust load is generated on the roller, the roller may slide in the axial direction and the friction may increase. It is therefore desired to suppress the generation of the thrust load as much as possible.